The present invention relates to integrated circuits, and more particularly to fabrication of microelectronic assemblies that include integrated circuits, interconnect substrates, and possibly other components.
Integrated circuits (ICs) are fragile devices assembled and interconnected to form preferably small, inexpensive, reliable assemblies with densely packed circuitry. The ICs can be interconnected through intermediate substrates such as printed circuit boards (PCBs) or interposers. Such assemblies often involve hybrid technologies: a highly precise, semiconductor technology is used to form densely packed ICs in semiconductor substrates; while coarser and cheaper technology is used to form PCBs in ceramic, glass or organic substrates. Each technology requires its own equipment and expertise, sometimes obtained through cooperation of different manufacturers.
FIG. 1 shows an exemplary assembly (exploded view) with two die 110, an interposer (ITP) 120 and a PCB 130. Each die 110 is a semiconductor IC formed initially in a semiconductor wafer (not shown) and later separated from the wafer. The die have contact pads 110C attached to the interposer's contact pads 120C.T (e.g. by a thin solder layer, not shown, formed on pads 110C or 120C.T). The interposer has bottom contact pads 120C.B attached to contact pads 130C of PCB 130, e.g. by larger solder balls 140. The interposer's interconnect lines 120L interconnect contact pads 120C.T, 120C.B in a desired pattern. The PCB's interconnect lines 130L interconnect contact pads 130C. Since the PCB is made by coarser technology, the contact pads 130C cannot be as small and dense as the die's contact pads 110C, so interposer 120 “redistributes” the connections: the interposer is made in a silicon substrate by IC technology, and its top contact pads 120C.T match the die's contact pads, and the bottom contact pads 120C.B match the PCB's contact pads. (The interposer may perform other functions in addition to redistribution of the connections, e.g.: the interposer may absorb some of the thermal expansion differences between the PCB and the die, thus relieving the stress on the die's and PCB's contact pads; and may absorb some of the heat to prevent overheating (overheating is a common cause of IC assembly failure).)
The interposer should be thin in order to reduce the vertical extent (and hence the length) of interconnects 120L—reduced length reduces parasitic inductances and capacitances, and hence provides higher circuit speed and lower power consumption. 100 micron thickness has been reported for silicon interposers, and lower thickness are contemplated. However, a thin interposer is fragile and cannot absorb much heat during manufacturing. Also, a thin interposer is easily warped, making it difficult to attach the interposer to the PCB because the solder balls 140 are at different heights due to warpage. Therefore, interposers have been strengthened by means of a temporary “handle” wafer which is temporarily bonded to one side of the interposer (by an adhesive) while the other side of the interposer is processed to form suitable circuitry. The handle wafer is then debonded. Regrettably, bonding and debonding complicate the manufacturing, increasing the cost and reducing the yield, and requiring a careful selection of the bonding adhesive capable to withstand the processing conditions and then be easily dissolved in debonding (dissolving can be replaced by laser oblation for some adhesives); see e.g. U.S. Pat. No. 7,960,840 (Bonifield et al., Jun. 14, 2011). These complexities are clearly undesirable.
Another option is to manufacture an interposer 120 as a thin, top layer in a thick substrate, then attach the die to this layer, and then thin the substrate from the bottom side. Solder bumps 140 can be attached after thinning. Before thinning, the die can be encapsulated in a molding compound (e.g. epoxy resin) to strengthen the structure, keep the interposer flat (not warped), and aid in heat dissipation. Alternative techniques are desirable.